Silicon Anode for a Rechargeable Battery

ABSTRACT

An electrode and electrode assembly, for example for use as an anode in a lithium-ion rechargeable cell that uses silicon or silicon-based elements of specific dimensions and geometry as its active material, is provided, as well as methods for manufacturing the same. The active silicon or silicon-based material may include fibres, sheets, flakes, tubes or ribbons, for example.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/599,034, filed Feb. 26, 2010, which is a national stage applicationunder 35 U.S.C. 371 of International Patent ApplicationPCT/GB2008/001604, filed Feb. 9, 2008, which claims the benefit ofpriority of United Kingdom patent application no. 0709165.5, filed May11, 2007, each of which is hereby incorporated herein by reference inits entirety

FIELD

This invention relates to an electrode for a rechargeable battery cellthat uses silicon or a silicon-based material as its active ingredient,in particular although not exclusively for use as the anode in alithium-ion battery cell.

TECHNICAL BACKGROUND

The recent increase in the use of portable electronic devices such asmobile telephones and notebook computers has created a need for smaller,lighter, longer lasting rechargeable batteries to provide the power tothe above mentioned and other battery powered devices. During the 1990s,lithium rechargeable batteries, specifically lithium-ion batteries,became popular and, in terms of units sold, now dominate the portableelectronics marketplace. However, as more and more power hungryfunctions are added to the above mentioned devices (e.g. cameras onmobile phones), improved batteries that store more energy per unit massand per unit volume are required.

It is well known that silicon can be used as the active anode materialof a rechargeable lithium-ion electrochemical battery cell (see, forexample, Insertion Electrode Materials for Rechargeable LithiumBatteries, M. Winter, J. O. Besenhard, M. E. Spahr, and P. Novuk in Adv.Mater. 1998, 10, No. 10). The basic composition of a conventionallithium-ion rechargeable battery cell is shown in FIG. 1 including agraphite-based anode electrode, the component to be replaced by thesilicon-based anode. The battery cell includes a single cell but mayalso include more than one cell.

The battery cell generally comprises a copper current collector for theanode 10 and an aluminium current collector for the cathode 12 which areexternally connectable to a load or to a recharging source asappropriate. A graphite-based composite anode layer 14 overlays thecurrent collector 10 and a lithium containing metal oxide-basedcomposite cathode layer 16 overlays the current collector 12. A porousplastic spacer or separator 20 is provided between the graphite-basedcomposite anode layer 14 and the lithium containing metal oxide-basedcomposite cathode layer 16 and a liquid electrolyte material isdispersed within porous plastic spacer or separator 20, the compositeanode layer 14 and the composite cathode layer 16. In some cases, theporous plastic spacer or separator 20 may be replaced by a polymerelectrolyte material and in such cases the polymer electrolyte materialis present within both the composite anode layer 14 and the compositecathode layer 16.

When the battery cell is fully charged, lithium has been transportedfrom the lithium containing metal oxide via the electrolyte into thegraphite-based layer where it reacts with the graphite to create thecompound, LiC₆. The graphite, being the electrochemically activematerial in the composite anode layer, has a maximum capacity of 372mAh/g. It will be noted that the terms “anode” and “cathode” are used inthe sense that the battery is placed across a load.

It is generally believed that silicon, when used as an active anodematerial in a lithium-ion rechargeable cell, provides a significantlyhigher capacity than the currently used graphite. Silicon, whenconverted to the compound Li₂₁Si₅ by reaction with lithium in anelectrochemical cell, has a maximum capacity of 4,200 mAh/g,considerably higher than the maximum capacity for graphite. Thus, ifgraphite can be replaced by silicon in a lithium rechargeable batterythe desired increase in stored energy per unit mass and per unit volumecan be achieved.

Existing approaches of using a silicon or silicon-based active anodematerial in a lithium-ion electrochemical cell have failed to showsustained capacity over the required number of charge/discharge cyclesand are thus not commercially viable.

One approach disclosed in the art uses silicon in the form of a powder(say as particles or spherical elements with a 10 μm diameter), in someinstances made into a composite with or without an electronic additiveand containing an appropriate binder such as polyvinylidene difluoridecoated onto a copper current collector. However, this electrode systemfails to show sustained capacity when subjected to repeatedcharge/discharge cycles. It is believed that this capacity loss is dueto partial mechanical isolation of the silicon powder mass arising fromthe volumetric expansion/contraction associated with lithiuminsertion/extraction to and from the host silicon. In turn this givesrise to electrical isolation of the silicon elements from both thecopper current collector and themselves. In addition, the volumetricexpansion/contraction causes the spherical elements to be broken upcausing a loss of electrical contact within the spherical elementitself.

Another approach known in the art designed to deal with the problem ofthe large volume changes during successive cycles is to make the size ofthe silicon elements that make up the silicon powder very small, that isto use spherical particles that have diameters in the 1-10 nm range.This strategy assumes that the nano-sized elements can undergo the largevolumetric expansion/contraction associated with lithiuminsertion/extraction without being broken up or destroyed. However, thisapproach is problematic in that it requires the handling of very fine,nano-sized powder that may pose a health and safety risk and it does notprevent the electrical isolation of the spherical elements from both thecopper current collector and themselves as the silicon powder undergoesthe volumetric expansion/contraction associated with lithiuminsertion/extraction. Importantly, since a lithium-containing surfacefilm is typically created during lithium insertion and the lithium ionsthat make up this surface film are trapped and can not be removed duringthe deinstertion process, the large surface area of the nano-sizedelements can give introduce large irreversible capacity into thelithium-ion battery cell. In addition, the large number of small siliconparticles creates a large number of particle-to-particle contacts for agiven mass of silicon and these each have a contact resistance and maythus cause the electrical resistance of the silicon mass to be too high.The above problems have thus prevented silicon particles from becoming acommercially viable replacement for graphite in lithium rechargeablebatteries and specifically lithium-ion batteries.

In another approach described by Ohara et al. in Journal of PowerSources 136 (2004) 303-306 silicon is evaporated onto a nickel foilcurrent collector as a thin film and this structure is then used to formthe anode of a lithium-ion cell. However, although this approach givesgood capacity retention, this is only the case for very thin films (say˜50 nm) and thus these electrode structures do not give usable amountsof capacity per unit area. Increasing the film thickness (say >250 nm)causes the good capacity retention to be eliminated. The good capacityretention of these thin films is considered by the present inventors tobe due to the ability of the thin film to absorb the volumetricexpansion/contraction associated with lithium insertion/extraction fromthe host silicon without the film being broken up or destroyed. Also,the thin film has a much lower surface area than the equivalent mass ofnano-sized particles and thus the amount of irreversible capacity due tothe formation of a lithium-containing surface film is reduced. The aboveproblems have thus prevented a thin film of silicon on a metal foilcurrent collector from becoming a commercially viable replacement forgraphite in lithium rechargeable batteries and specifically lithium-ionbatteries.

In another approach described in U.S. Pat. No. 6,887,511, silicon isevaporated onto a roughened copper substrate to create medium-thicknessfilms of up to 10 μm. During the initial lithium ion insertion process,the silicon film breaks up to form pillars of silicon. These pillars canthen reversibly react with lithium ions and good capacity retention isachieved. However, the process does not function well with thicker filmsand the creation of the medium-thickness film is an expensive process,thus limiting this concept's commercially viability. Also, the pillaredstructure created by the break up of the film has no inherent porosityand thus the long terms capacity retention is questionable.

In another approach described in US2004/0126659, silicon is evaporatedonto nickel fibres which are then used to form the anode of a lithiumbattery. However this is found to provide an uneven distribution ofsilicon on the nickel fibres hence significantly affecting operation. Inaddition, these structures have a high ratio of nickel current collectormass to active silicon mass and thus do not give usable amounts ofcapacity per unit area or per unit mass.

A review of nano- and bulk-silicon-based insertion anodes forlithium-ion secondary cells has been provided by Kasavajjula et al (J.Power Sources (2006), doi:10.1016/jpowsour.2006.09.84), herewithincorporated by reference herein.

SUMMARY OF THE DISCLOSURE

The invention is set out in the independent claims.

Advantageously, some embodiments provide an electrode containing as itsactive material an interconnected array of high-aspect ratio silicon orsilicon-based elements. Cycle life is improved as the structure of theelements, in conjunction with an upper limit of the smallest dimensionof the elements, allows for accommodation of the volume expansionassociated with insertion/extraction (charging and discharging) of thesilicon or silicon-based elements while a lower limit on the smallestdimension controls the ratio of surface area for a given volume ofsilicon or silicon-based and thus minimises the surface-relatedirreversible capacity. At least one other dimension is chosensufficiently large such as to ensure multiple contacts between elementsfor good electronic conductivity.

The high-aspect ratio elements may be elongate, for example ribbon-likesuch that a first larger dimension is larger than the smallest dimensionand a second larger dimension is larger than the first larger dimension.High-aspect ratio elements may also be sheet-like or flake-like, whereinthe first and second larger dimensions are larger than the firstdimension but comparable to each other.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a lithium ion rechargeable cell includingan anode electrode in accordance with embodiments of the invention.

DETAILED DESCRIPTION

The invention is now described, by way of example only and withreference to the accompanying FIG. 1, schematically showing a lithiumion rechargeable cell including an anode electrode in accordance withembodiments of the invention.

It has been realised by the inventors that the above-mentioned problemsand drawbacks of the prior art may be addressed by carefully selectingthe dimensions and geometry of the silicon or silicon-based elementsthat are the active ingredient of an electrode for a rechargeablebattery. For elongate elements which have two comparable dimensionssmaller than a third dimension (referred to as fibres in the remainder),to a first approximation, the irreversible capacity loss is inverselyproportional to the diameter of the fibre. Similarly, for an elongatestructure for which one of the two smaller dimensions is larger, thanthe other one, for example twice as large or more as the smallerdimension (referred to below as a ribbon) and for a element which hastwo comparable largest dimension and a single dimension smaller thanthat (referred to as a sheet or flake below) the irreversible capacitycan be shown to be approximately inversely proportional to the thicknessof the ribbon or sheet (that is the smallest dimension), ignoring thesides of the ribbon or the sheet. Thus, for fibres, ribbons, flakes orsheets, a ten-fold decrease in the smallest dimension approximately isexpected to result in a ten-fold increase in the irreversible capacityloss. These considerations impose a lower limit on the smallestdimension for these structures if they are to be used as siliconelements in a composite electrode with limited irreversible capacityloss.

As discussed above, one significant problem in the use of silicon orsilicon-based materials as the active anode material for a lithium-ionrechargeable battery cell is the large volume changes associated withthe charging and discharging of the cell. The associated stresses leadto crack formation in bulk silicon, as described above. Experimentalwork on pillar-shaped silicon substrates has shown that silicon pillarsof close to 1 micrometer diameter (approximately 0.8 micrometer) can beformed which can accommodate the volume changes without cracking [MinoGreen, Elizabeth Fielder, Bruno Scrosati, Mario Wachtler and JudithSerra Moreno, “Structured Silicon Anodes for Lithium BatteryApplications”, Electrochemical and Solid-State Letters:6,A75-A79(2003).] Furthermore, experimental work on silicon plates hasshown that even in thick plates (350 microns thickness) stress fractureshave a characteristic length of 10 microns.

Based on the foregoing considerations, the smallest dimension of siliconor silicon-based elements in a electrode in accordance with anembodiment of the invention may be in the range of 0.08 to 1 μm,preferably 0.2 μm to 0.3 μm or within the range therebetween. To furtherensure a favourable surface area to volume ratio, the second largestdimension should be at least two times as large as the smallestdimension.

Another consideration is the number of electrical interconnectionsbetween the elements. For elongate elements such as fibres or ribbons,the larger the largest dimension, the more likely the individual membersare to criss-cross each other and form multiple connections therebetween. Similarly, for sheet or flake-like members, the larger theflakes or sheets, the more likely they will be to mutually overlap.Moreover, the larger the one or two largest dimensions, the more mass ofsilicon will be arranged for a given surface area, further reducingirreversible capacity. Based on these considerations, the largest, orlargest two dimensions are chosen to be larger than ten times thesmallest dimension, preferable 100 or 200 times larger or within therange therebetween. The total length or largest dimension may be aslarge as 500 μm, for example.

It will be appreciated, of course, that any appropriate approach can beadopted in order to fabricate the silicon or silicon-based elementsdiscussed above.

For example, fibres can be manufactured by forming pillars on a suitablesilicon or silicon-based substrate and detaching these pillars to createfibres by a suitable method. Pillars of silicon can be manufactured asdescribed in PCT/GB2007/000211 or as described in U.S. application Ser.No. 10/049736.

Ribbons of silicon can be manufactured via a lithography process suchthat suitably shaped structure are made on a silicon or silicon-basedsubstrate and then detached from the substrate using a suitabledetachment method.

Sheets (or also flakes) may be manufactured using thin film depositionof silicon on poorly adhering substrates leading to detachable sheets ofsilicon. If the detachable sheet is broken up, flakes result.

Once the silicon or silicon-based elements have been manufactured theycan be used as the active material in a composite anode for lithium-ionelectrochemical cells. To fabricate a composite anode, the elements canbe mixed with polyvinylidene difluoride and made into a slurry with acasting solvent such as n-methyl pyrrolidinone. This slurry can then beapplied or coated onto a metal foil or other conducting substrate forexample physically with a blade or in any other appropriate manner toyield a coated film of the required thickness and the casting solvent isthen evaporated from this film using an appropriate drying system whichmay employ elevated temperatures in the range of 50 degrees C. to 140degrees C. to leave the composite film free or substantially fromcasting solvent. The resulting composite film has a porous structure inwhich the mass of silicon or silicon-based elements is typically between70 percent and 95 percent. The composite film will have a percentagepore volume of 10-30 percent, preferably about 20 percent.

Fabrication of the lithium-ion battery cell thereafter can be carriedout in any appropriate manner for example following the generalstructure shown in FIG. 1 but with a silicon or silicon based activeanode material rather than a graphite active anode material. For examplethe silicon elements-based composite anode layer is covered by theporous spacer 18, the electrolyte added to the final structuresaturating all the available pore volume. The electrolyte addition isdone after placing the electrodes in an appropriate casing and mayinclude vacuum filling of the anode to ensure the pore volume is filledwith the liquid electrolyte.

A particular advantage of the approach described herein is that largesheets of silicon-based anode can be fabricated and then rolled orstamped out subsequently as is currently the case in graphite-basedanodes for lithium-ion battery cells meaning that the approach describedherein can be retrofitted with the existing manufacturing capability.

It will be appreciated, of course, that any appropriate approach can beadopted in order to arrive at the approaches and apparatus describedabove. For example the element manufacture can comprise any of asuitable method employed in the silicon processing industry. The cathodematerial can be of any appropriate material, typically a lithium-basedmetal oxide material. The elements can have any appropriate dimensionand can for example be pure silicon or doped silicon or othersilicon-based material such as a silicon-germanium mixture or any otherappropriate mixture.

The above description is by way of example only and not intended to belimiting on the scope of the claimed subject matter which is intended tocover any such modifications, juxtapositions or alterations of theabove-described embodiments as may appear to the skilled person. Forexample, although the specific description has been presented in termsof silicon as an electrode material, other silicon-based materials maybe employed in place of undoped silicon, such as doped silicon, forexample SiGe.

The present invention resulted from work undertaken under a jointresearch agreement between Nexeon Ltd and Imperial Innovations ltd inthe field of batteries, rechargeable cells and associated energy storagedevices.

1-14. (canceled)
 15. An electrode comprising an active materialcomprising a plurality of silicon or silicon-based elements, eachelement having a first dimension in the range of 0.08 μm to 0.3 μm and asecond dimension oriented generally transverse to the first dimension,the second dimension being at least five times as large as the firstdimension, wherein the elements form an interconnected array, whereinthe elements are capable of lithium insertion and removal, and whereinthe elements are selected from the group consisting of hollow tubes,ribbons and flakes.
 16. An electrode according to claim 15 wherein thefirst dimension is in the range of 0.2 μm to 0.3 μm.
 17. The electrodeaccording to claim 15, wherein the second dimension is at least tentimes as large as the first dimension.
 18. The electrode according toclaim 15, wherein the elements have a third dimension orientedtransverse to each of the first and second dimensions, the thirddimension being at least ten times as large as the first dimension. 19.The electrode according to claim 15, wherein the elements have a thirddimension oriented transverse to each of the first and seconddimensions, the third dimension being at least ten times as large as thefirst dimension.
 20. The electrode according to claim 15 wherein theelements have a third dimension oriented transverse to each of the firstand second dimensions, the third dimension being at least 100 times aslarge as the first dimension.
 21. The electrode according to claim 15,wherein the elements have a largest dimension that is no larger than 500μm.
 22. The electrode according to claim 15, wherein the elements areelongate.
 23. The electrode according to claim 15, wherein the siliconor silicon-based elements are hollow tubes.
 24. The electrode accordingto claim 15, wherein the silicon or silicon-based elements are ribbons.25. The electrode according to claim 15, wherein the silicon orsilicon-based elements are flakes.
 26. The electrode according to claim15, wherein the elements of the interconnected array form multipleconnections therebetween.
 27. The electrode according to claim 15,wherein the elements of the interconnected array criss-cross oneanother.
 28. The electrode according to claim 15, wherein the activematerial has a pore volume between 10% and 30%.
 29. The electrodeaccording to claim 15, wherein the active material has a pore volume,the pore volume being saturated with a liquid electrolyte.
 30. Anelectrochemical cell comprising the electrode as claimed in claim 15configured as an anode, and a cathode comprising an active materialcomprising a lithium-based metal oxide.
 31. A lithium-ion rechargeablebattery comprising the electrochemical cell according to claim
 30. 32. Adevice comprising the electrochemical cell according to claim 30, theelectrochemical cell being configured to power the device.